Valve actuators are configured to operate valves that regulate or control the flow of a fluid through a passageway by opening, closing, or partially obstructing the passageway. A number of forces and pressures may act on the valve from the fluid in the passageway, and, depending on the environment in which a valve system is implemented, the amount of torque required to actuate the valve may be significant. In some environments, such as valves that are implemented in a seawater system (e.g., a marine vessel), a valve may get stuck when the valve is infrequently operated. For example, barnacles and other obstructions may form over time, which may affect how much torque is required to actuate the valve when it is necessary to do so. Thus, in many instances, it is useful to measure and monitor the torque applied to the valve at any given time during operation of the valve.
Automated valve actuators typically include an electric motor. Some valve actuators may include an electric motor in the form of a direct current (DC) motor(s). For DC motors, measuring torque is relatively simple. Since the current that is drawn by the DC motor may be taken as an indirect measurement of the torque being produced by the DC motor, a measurement of the current drawn by the DC motor may be used to derive the torque, and such a measurement is relatively easy to make.
However, many valve actuators, such as those that are implemented in marine environments like ocean liners and military ships, use one or more alternating current (AC) motors as the drive mechanism for operating the associated valve. For AC motors, one cannot simply measure the current drawn by the AC motor. For instance, as the torque applied by the AC motor increases, the absolute value of the AC current may not change significantly, possibly making it difficult to derive torque from a simple measurement of AC current.
To this end, various systems have been developed to measure torque applied to a valve by an AC motor. Some systems involve the use of mechanical-based sensors (e.g., strain gauges, brackets, etc.) that measure the torque applied to the valve based on output of the mechanical sensors. However, adding multiple mechanical-based sensors to a valve actuator can undesirably increase the weight and cost of the actuator. Electrical sensors have been developed to monitor torque produced by AC motors. However, conventional electrical sensors can be complex in design, making for a system that is relatively difficult and costly to manufacture.